U.S. patent application number 14/410797 was filed with the patent office on 2015-06-18 for power tool.
This patent application is currently assigned to Hitachi Koki Co., Ltd.. The applicant listed for this patent is Hitachi Koki Co., Ltd.. Invention is credited to Kazuhiko Funabashi, Yuki Horie, Nobuhiro Takano.
Application Number | 20150171654 14/410797 |
Document ID | / |
Family ID | 49209517 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150171654 |
Kind Code |
A1 |
Horie; Yuki ; et
al. |
June 18, 2015 |
POWER TOOL
Abstract
A power tool includes: a secondary battery; a drive unit; a
detecting unit; and a control unit. The secondary battery has
positive and negative terminals across which a battery voltage is
developed. The drive unit is connected to the secondary battery.
The detecting unit is configured to detect a current value and a
voltage value supplied from the secondary battery to the drive
unit. The control unit is configured to control an effective
voltage and an effective current applied to the drive unit
depending on a load imposed thereon to fall within an allowable
power dissipation value.
Inventors: |
Horie; Yuki; (Hitachinaka,
JP) ; Funabashi; Kazuhiko; (Hitachinaka, JP) ;
Takano; Nobuhiro; (Hitachinaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi Koki Co., Ltd. |
Tokyo |
|
JP |
|
|
Assignee: |
Hitachi Koki Co., Ltd.
Tokyo
JP
|
Family ID: |
49209517 |
Appl. No.: |
14/410797 |
Filed: |
August 29, 2013 |
PCT Filed: |
August 29, 2013 |
PCT NO: |
PCT/JP2013/005128 |
371 Date: |
December 23, 2014 |
Current U.S.
Class: |
173/2 |
Current CPC
Class: |
H02J 7/0063 20130101;
B25F 5/00 20130101 |
International
Class: |
H02J 7/00 20060101
H02J007/00; B25F 5/00 20060101 B25F005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2012 |
JP |
2012-189921 |
Claims
1. A power tool comprising: a secondary battery having positive and
negative terminals across which a battery voltage is developed; a
drive unit connected to the secondary battery; a detecting unit
configured to detect a current value and a voltage value supplied
from the secondary battery to the drive unit; and a control unit
configured to control an effective voltage and an effective current
applied to the drive unit depending on a load imposed thereon to
fall within an allowable power dissipation value.
2. The power tool according to claim 1, wherein the control unit
executes either one of a first control and a second control,
wherein the first control continuously applies the battery voltage
to the drive unit, and the second control converts the battery
voltage, the converted battery voltage having an effective voltage
different from the battery voltage, at least one converted battery
voltage being applied to the drive unit.
3. The power tool according to claim 1, wherein the battery voltage
is different from a rated voltage of the drive unit.
4. The power tool according to claim 1, further comprising a
current interrupting unit configured to interrupt current flowing
to the drive unit in response to an alert signal alerting that the
secondary battery has an error.
5. The power tool according to claim 4, wherein the alert signal
alerts that the secondary battery is about to become at least one
of an over-discharge state, an over-current state in which
overcurrent flows from the secondary battery, and a
high-temperature state in which a temperature of the secondary
battery exceeds a prescribed limit temperature.
6. The power tool according to claim 1, wherein the control unit is
further configured to change current from the secondary battery
into different values step-by-step depending on the battery voltage
by outputting a square-wave signal of prescribed frequency as a
chopping control signal and varying a duty cycle of the chopping
control signal.
7. The power tool according to claim 6, further comprising a
current interrupting unit configured to interrupt current flowing
to the drive unit in response to an alert signal alerting that the
secondary battery has an error, wherein the control unit is further
configured to change the current from the secondary battery by
turning the current interrupting unit on and off in response to the
chopping control signal.
8. The power tool according to claim 1, wherein the secondary
battery comprises a lithium-ion battery cell.
Description
TECHNICAL FIELD
[0001] The present invention relates to a power tool that is
powered by secondary batteries.
BACKGROUND ART
[0002] Battery packs housing secondary batteries are commonly used
to power cordless power tools (For example, refer to Japanese
Patent Application Publication No. 2011-136405). A power tool
includes a motor that produces an output suited to the application
of the power tool, and a control circuit configured of a
field-effect transistor (FET), for example, that controls the
electric current supplied to the motor. Conventionally, power tools
have been provided with a specialized battery pack having a voltage
and capacity suited to the output of the motor and FET. As a
result, a plurality of types of battery packs exists for a
plurality of types of power tools.
CITATION LIST
Patent Literature
[0003] Japanese Patent Application Publication No. 2011-136405
DISCLOSURE OF INVENTION
Solution to Problem
[0004] As described above, battery packs for conventional power
tools have specialized rather than general-purpose battery packs
and thus cannot be used for power tools other than the type for
which they were made.
[0005] For example, a battery pack having an output of 18 V cannot
be used with a power tool designed for a 14.4-V battery pack.
Accordingly, a manufacturer that produces multiple types of power
tools must prepare and provide individual battery packs
respectively compatible with the individual power tools. At the
same time, the consumer must store and keep track of each purchased
power tool with the battery pack designed for use with that power
tool.
[0006] Consequently, employing specialized battery packs for each
power tool is a factor that inhibits low cost since the
manufacturer must tack on the cost of the battery pack to each
newly purchased power tool. Storing and managing each battery pack
with its respective power tool can also be confusing and
inconvenient for the consumer.
[0007] Further, since a specialized battery pack designed for an
existing power tool cannot be used with a newly purchased power
tool, the use of specialized battery packs is clearly inconvenient
and uneconomical and does not meet the needs of the times. From a
consumer's perspective, it would be desirable to have a singly
battery pack that can be used universally with various types of
power tools.
[0008] In view of the foregoing, it is an object of the present
invention to provide a power tool enabling various types of battery
packs to be used universally. In order to attain the above and
other objects, the present invention provides a power tool
including: a secondary battery having positive and negative
terminals across which a battery voltage is developed; a drive unit
connected to the secondary battery; a detecting unit configured to
detect a current value and a voltage value supplied from the
secondary battery to the drive unit; and a control unit (control
FET, for example) configured to control an effective voltage and an
effective current applied to the drive unit depending on a load
imposed thereon to fall within an allowable power dissipation
value.
[0009] With this configuration, the effective current that is
consistent with the battery voltage of the battery is applied to
the drive unit or control FET so as to fall within the allowable
power dissipation value of the drive unit or control FET.
Consequently, the power tool is capable of supplying a high-power
output within the ability allowable for the power tool and
promoting greater work efficiency when a battery pack having a
higher battery voltage than that of a battery pack specifically
designed for the power tool is used.
[0010] Preferably, the control unit executes either one of a first
control and a second control, wherein the first control
continuously applies the battery voltage to the drive unit, and the
second control converts the battery voltage, the converted battery
voltage having an effective voltage different from the battery
voltage, at least one converted battery voltage being applied to
the drive unit.
[0011] With this configuration, the control unit executes a
plurality of controls by applying a plurality of powers having
different effective voltages to the motor, including a control that
directly applies the battery voltage and the current of the
secondary battery to the motor. Therefore, it is possible to
execute an optimal control suited for the drive situation of the
drive unit.
[0012] Preferably, the battery voltage is different from a rated
voltage of the drive unit. Consequently, the battery pack housing a
secondary battery available for the power tool is not limited to a
battery pack specifically designed for the power tool. Various
types of battery packs having different battery voltages and
capacities can be used for the power tool.
[0013] Preferably, the driving tool further includes a current
interrupting unit configured to interrupt current flowing to the
drive unit in response to an alert signal alerting that the
secondary battery is about to become an abnormal state. The alert
signal alerts that the secondary battery is about to become at
least one of an over-discharge state, an over-current state in
which overcurrent flows from the secondary battery, and a
high-temperature state in which a temperature of the secondary
battery exceeds a prescribed temperature.
[0014] Preferably, the control unit is further configured to change
current from the secondary battery into different values
step-by-step depending on the battery voltage by outputting a
square-wave signal of prescribed frequency as a chopping control
signal and varying a duty cycle of the chopping control signal. The
control unit is further configured to change the current from the
secondary battery by turning the current interrupting unit on and
off in response to the chopping control signal. Because the current
interrupting unit is also used for the chopping control, it is
possible to suppress the increase of the number of the components
included in the power tool even when the chopping control function
is added to the power tool.
[0015] Specifically, every well-known battery can be included in
the secondary battery, and the developed secondary battery is also
applicable in future. However, it is preferable to use a
lithium-ion battery at the time of the present application.
Advantageous Effects of Invention
[0016] The present invention enables various battery packs having
different output voltages to be used with the same power tool by
keeping power loss, calculated as the product of the output voltage
from the mounted battery pack and the current flowing through the
current interrupting unit (control FET, for example; W (watts)=A
(current)*V (voltage)), from not exceeding the rated power loss of
the current interrupting unit, enabling battery packs to be used
universally. The structure of the present invention can also be
simplified by using the current interrupting unit in chopping
control to cut off the current flowing to the drive unit when the
temperature of the secondary batteries exceeds a prescribed
temperature (high-temperature state).
[0017] Since the rotational speed of the motor is generally greater
at higher voltages, a larger load tends to be applied to the motor
during startup and the locked rotor current when rotation of the
motor is halted tends to increase.
[0018] Therefore, when using an 18-V battery to power a power tool
equipped with a 14.4-V motor, for example, the power tool according
to the present invention reduces the effective voltage of the
battery through chopping control during startup in order to prevent
the motor from rotating too fast (this control is referred to as a
"soft start"). This prevents components in the drive unit of the
motor and the like from becoming damaged. Electric current must
also be restricted through a chopping control technique when the
motor locks to prevent damage to the motor.
[0019] On the other hand, it is not necessary to limit current
through chopping control or the like anytime other than startup
(when the battery voltage is high) and motor lock (when discharge
is cut off) because the load on the drive unit of the motor and the
like is relatively small, provided that power dissipation in the
drive unit is less than the maximum allowable. Thus, the operator
can comfortably use the power tool at the maximum power possessed
by the battery or motor. With the power tool according to the
present invention described above, a battery pack having a
different battery voltage from the voltage of the power tool can be
connected to the power tool to achieve suitable operations without
applying excessive load to the power tool.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a circuit diagram of a power tool and a battery
pack according to an embodiment of the present invention.
[0021] FIG. 2 is a flowchart illustrating operations of the power
tool shown in FIG. 1.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] Next, a power tool 1 according to a preferred embodiment of
the present invention will be described whole referring to the
accompanying drawings. FIG. 1 shows a circuit diagram of the power
tool 1, and a battery pack 2 mounted on the power tool 1.
[0023] The battery pack 2 houses a lithium-ion battery 6, a battery
protection circuit 7, a thermistor 8, and a resistor R1. The
lithium-ion battery 6 is configured of a plurality of battery cells
connected in series. The battery protection circuit 7 is connected
to the lithium-ion battery 6. The thermistor 8 is disposed near to
or in contact with the lithium-ion battery 6 and detects the
temperature of the same. Output from the thermistor 8 is inputted
into the battery protection circuit 7.
[0024] Normally, the battery pack 2 provided with the power tool 1
is a specialized battery pack designed for a motor 3 (described
later) provided in the power tool 1. However, in the preferred
embodiment the battery pack 2 connected to the power tool 1 need
not be a specialized battery pack that is compatible with the power
tool 1, but may be another type of battery pack having a different
output voltage from the specialized battery pack. As an example, if
the rated voltage of the motor 3 is 14.4 V, then the specialized
battery pack 2 provided with the power tool 1 may be configured of
four battery cells connected in series, where each cell is 3.6 V.
However, in order to improve the versatility of battery packs, the
power tool 1 can be connected to battery packs having an output
voltage other than 14.4 V, such as 18 V, 25 V, or 36 V. In the
preferred embodiment, the rated voltage of the motor 3 is 14.4 V.
The battery pack 2 connected to the power tool 1 is configured of
five lithium-ion battery cells of 3.6 V per cell connected in
series for a total battery voltage of 18 V. However, the battery
pack 2 used in the present invention is not limited to a
lithium-ion battery, such as a nickel-cadmium battery, nickel-metal
hydride battery, and lead-acid battery. Of the examples of
secondary batteries given above, a lithium-ion battery is
preferable for its high energy density.
[0025] The battery protection circuit 7 monitors the battery
voltage, discharge current, and temperature of each cell in the
lithium-ion battery 6. The battery protection circuit 7 determines
that the lithium-ion battery 6 is in an over-discharge state when
the voltage of any cell drops below a prescribed value. Upon
detecting an over-discharge state, the battery protection circuit 7
outputs an alert signal from a battery shutdown terminal 9 of the
battery pack 2. The battery protection circuit 7 also determines
that an over-current condition has occurred when the discharge
current from the lithium-ion battery 6 exceeds a prescribed value,
and outputs the same alert signal through the battery shutdown
terminal 9. The thermistor 8 functions to detect the temperature of
the lithium-ion battery 6 and to input the detection results into
the battery protection circuit 7. If the battery protection circuit
7 determines that the temperature of the lithium-ion battery 6
exceeds a prescribed value (i.e., is too hot), the battery
protection circuit 7 outputs the same alert signal through the
battery shutdown terminal 9. The battery protection circuit 7 also
detects an over-charge condition and the like when the lithium-ion
cell set 6 is being charged and outputs an alert signal to the
charging device for halting the charging operation.
[0026] The power tool 1 has a positive (+) terminal and a negative
(-) terminal that connect to the corresponding positive and
negative terminals of the battery pack 2. The power tool 1 and
battery pack 2 both possess a battery shutdown terminal 9 that
connect to each other when the battery pack 2 is mounted on the
power tool 1.
[0027] The power tool 1 has a motor 3, a trigger switch 4, and a
control FET 5 that are connected in series between the positive and
negative terminals of the power tool 1. When the battery pack 2 is
mounted on the power tool 1 and both the trigger switch 4 and
control FET 5 are on, the lithium-ion battery 6 supplies power to
the motor 3 for driving the motor 3 to rotate.
[0028] The power tool 1 is also provided with a three-terminal
regulator 10. The three-terminal regulator 10 outputs a constant
voltage of 5 V based on the battery voltage supplied from the
lithium-ion battery 6. The 5-V constant voltage is used to power a
microcomputer 11 and a storage device 12 described later.
Capacitors C1 and C2 are connected to the three-terminal regulator
10 for preventing circuit oscillation.
[0029] The power tool 1 further includes a microcomputer 11 and a
storage device 12. As described above, the output terminal of the
three-terminal regulator 10 is connected to the VDD terminal of the
microcomputer 11 across which a voltage of 5 V is applied. The
microcomputer 11 is in an operating state when the 5-V is applied
to the VDD terminal. The 5-V voltage is similarly applies to the
storage device 12 from the output terminal of the three-terminal
regulator 10, placing the storage device 12 in an operating state.
The storage device 12 is connected to the microcomputer 11, and the
microcomputer 11 can read data stored in the storage device 12 and
temporarily store this data in RAM (not shown) provided in the
microcomputer 11. The microcomputer 11 also includes a timer (not
shown).
[0030] The storage device 12 stores a first allowable power
dissipation W1 (also called the "rated power loss") for the motor 3
and the control FET 5 during a low load period when the current
supplied to the motor 3 is small; a second allowable power
dissipation W2 for the motor 3 and the control FET 5 during a high
load period when the current supplied to the motor 3 is large; and
a third allowable power dissipation W3 for the motor 3 and the
control FET 5 during a continuous drive period when the driving
time of the motor 3 exceeds a prescribed time. Each of the
allowable power dissipations W1, W2, and W3 is set to a value that
does not exceed an allowable power dissipation W determined for the
motor 3 and the control FET 5. The allowable power dissipation for
the motor 3 and control FET 5 can be expressed as the product of
the current A flowing through the motor 3 (or control FET 5) and
the voltage of the lithium-ion battery 6 (W=A*V).
[0031] The power tool 1 is also provided with a battery voltage
detection circuit 13 configured of resistors R2 and R3 connected in
series. The battery voltage detection circuit 13 is connected in
parallel to the lithium-ion battery 6. The resistors R2 and R3
divide the battery voltage from the lithium-ion battery 6 so that
the voltage inputted into the microcomputer 11 is a divided voltage
corresponding to the battery voltage of the lithium-ion battery 6.
The microcomputer 11 temporarily stores data in internal RAM (not
shown) representing the input battery voltage. A current detection
circuit 14 is connected to the microcomputer 11 for detecting
current flowing to the motor 3. The current detection circuit 14
outputs the detected current value to the microcomputer 11, and the
microcomputer 11 temporarily stores data in RAM representing the
current flowing to the motor 3. The current detection circuit 14 is
configured of resistors (not shown).
[0032] An shutdown circuit 15 is connected to the battery shutdown
terminal 9 on the power tool 1 side. The shutdown circuit 15 is
also connected to the gate of the control FET 5 through a resistor
R8. The shutdown circuit 15 is configured of an FET 15a, a resistor
R4 connected to the gate of the FET 15a, and a resistor R5
connected to between the gate and source of the FET 15a. The drain
of the FET 15a is connected to the gate of the control FET 5
through the resistor R8.
[0033] The power tool 1 is also provided with a chopper circuit 16
between a chopping control terminal of the microcomputer 11 and the
control FET 5. The chopper circuit 16 is configured of an FET 16a,
and resistors R6 and R7. The resistor R6 is connected between the
chopping control terminal of the microcomputer 11 and the FET 16a.
The resistor R7 is connected between the gate and source of the FET
16a. The FET 16a of the chopper circuit 16 is connected to the gate
of the control FET 5 via the resistor R8. In other words, both the
shutdown circuit 15 and the chopper circuit 16 can turn the control
FET 5 on and off. That is, the control FET 5, which is used to
interrupt current flowing to the motor 3 in response to an alert
signal, also plays a role in chopping control for the battery
voltage.
[0034] In the preferred embodiment, chopping control is a technique
in which the control FET 5 turns the battery voltage (or current)
supplied from the lithium-ion battery 6 on and off in order to
apply a square-wave current to the motor 3 or control FET 5. Thus,
chopping control can regulate the effective voltage/current applied
to the motor 3 and control FET 5. The ON duration and OFF duration
of the control FET 5 is determined by a square-wave chopping
control signal of a prescribed frequency supplied from the chopping
control terminal of the microcomputer 11. In other words, the
effective voltage applied to the motor 3 and the control FET 5 is
determined by the duty cycle and is lower than the battery voltage
produced by the lithium-ion battery 6. Accordingly, chopping
control can be used to generate a voltage/current that is
compatible with the motor 3 and control FET 5 in the power tool 1,
even when the battery pack connected to the power tool 1 outputs a
higher voltage than the battery pack designed for use with the
motor 3.
[0035] Resistors R9 and R10 are connected in series between the
positive terminal of the motor 3 and the source of the control FET
5. The gate of the control FET 5 is connected to a connection point
between the resistors R9 and R10. While an alert signal is not
being outputted and while chopping control is not being performed,
the control FET 5 is maintained in an ON state by the current
flowing through the resistors R9 and R10.
[0036] Next, the operations of the power tool 1 and battery pack 2
having the above configurations will be described while referring
to the flowchart in FIG. 2.
[0037] When the battery pack 2 is mounted on the power tool 1 and a
power switch (not shown) on the power tool 1 is turned on, the
three-terminal regulator 10 in the power tool 1 generates a 5-V
supply voltage based on the battery voltage supplies from the
lithium-ion battery 6 and applies this voltage to the microcomputer
11 and storage device 12. When the three-terminal regulator 10
applies this voltage, the microcomputer 11 and storage device 12
enter an operable state, and the microcomputer 11 begins a control
process. Under normal conditions, the battery protection circuit 7
does not output an alert signal (low level signal) at this time.
Accordingly, the FET 15a of the shutdown circuit 15 is turned on,
turning on the control FET 5.
[0038] At the beginning of the control process, in S1 the
microcomputer 11 detects the battery voltage of the lithium-ion
battery 6 based on input data from the battery voltage detection
circuit 13. In S2 the microcomputer 11 determines whether the
trigger switch 4 is on. If the trigger switch 4 is off (S2: NO),
the microcomputer 11 returns to S1 and continues to detect the
battery voltage while waiting for the trigger switch 4 to turn on.
However, if the trigger switch 4 is on (S2: YES), in S3 the
microcomputer 11 sets a voltage value a volts that will be applied
to the motor 3 to the battery voltage detected in S1. In this
example, the microcomputer 11 detects the battery voltage as 18
V.
[0039] Next, the microcomputer 11 reads the first allowable power
dissipation W1 from the storage device 12 and temporarily stores
this data in its internal RAM. The microcomputer 11 calculates a
current value x amperes based on the first allowable power
dissipation W1 read from the storage device 12 and the voltage
value a volts set in S3. The current value x amperes can be found
from the following equation.
<current value x amperes>=<first allowable power
dissipation W1>/<voltage value a volts>
[0040] The current value x amperes calculated above denotes the
maximum current that can be applied to the motor 3 or control FET 5
during a low load period.
[0041] In S4 the microcomputer 11 compares the current value x
amperes calculated according to the above equation with the value
of current detected by the current detection circuit 14 to
determine whether the detected current exceeds the current value x
amperes. If the current detected in the current detection circuit
14 exceeds the current value x amperes (S4: YES), indicating that
the motor 3 of control FET 5 is in a high load state rather than a
low load state, then the microcomputer 11 calculates an effective
voltage to apply to the motor 3 or control FET 5 that is consistent
with the current actually flowing to the motor 3 and control FET 5
in order to stay within the second allowable power dissipation W2.
To this end, in S5 the microcomputer 11 performs chopping control
to set the effective voltage applied to the motor 3 to a voltage
value b volts. On the other hand, if the current value detected by
the current detection circuit 14 is less than the current value x
amperes (S4: NO), indicating that power loss in the motor 3 and
control FET 5 is within the first allowable power dissipation W1,
the microcomputer 11 returns to S2 and allows operations of the
power tool 1 to continue. Hence, chopping control is not performed
when load on the motor 3 is low, i.e., when the current flowing to
the motor 3 is small, since there is no need to limit current
applied to the control FET 5 at this time.
[0042] As described above, chopping control is the process of
controlling the effective voltage applied to the motor 3 or control
FET 5 by outputting a square-wave signal (chopping control signal)
of prescribed frequency from the chopping control terminal of the
microcomputer 11 for turning the control FET 5 on and off.
Specifically, when the chopping control signal is high level, the
FET 16a of the chopper circuit 16 turns on, which turns the control
FET 5 on. As a result, the battery voltage from the lithium-ion
cell set 6 is applied to the motor 3 while the chopping control
signal remains at high level. However, when the microcomputer 11
outputs a low level chopping control signal, the FET 16a of the
chopper circuit 16 is turned off and, consequently, the control FET
5 is turned off. As a result, a voltage is no longer applied to the
motor 3 while the chopping control signal remains at low level. By
varying the duty cycle of the chopping control signal, it is
possible to change the effective voltage applied to the motor 3 or
control FET 5.
[0043] When the battery protection circuit 7 of the battery pack 2
outputs an alert signal (low level), this signal is applied to the
chopper circuit 16 through the battery shutdown terminal 9 on the
power tool 1 side. The signal turns off the FET 16a of the chopper
circuit 16, thereby turning off the control FET 5. Consequently, an
electric current does not flow through the motor 3 and control FET
5, halting operations of the power tool 1. With the control FET 5
serving also as an FET for use in chopping control in the preferred
embodiment, the number of parts required for executing chopping
control can be minimized.
[0044] The voltage value b volts set as the effective voltage in S5
is found from the second allowable power dissipation W2 stored in
the storage device 12 for a high load condition of the motor 3 and
the value of current detected by the current detection circuit 14.
The microcomputer 11 reads this second allowable power dissipation
W2 from the storage device 12 and temporarily stores the data in
internal RAM. In S5 the microcomputer 11 calculates the voltage
value b volts according to the following equation.
<voltage value b volts>=<second allowable power
dissipation W2>/<detected current value>
[0045] In this way, a large load current can be limited by reducing
the effective voltage applied to the control FET 5. Therefore,
operations of the power tool 1 can continue without exceeding the
preset allowable power dissipation for the motor 3 and control FET
5, even when the load current is high.
[0046] In S6 the microcomputer 11 uses its internal timer to
determine whether a current greater than the current value x
amperes has been continuously applied for a prescribed time T or
greater. Since the value of the current is sampled at prescribed
intervals and stored in the RAM of the microcomputer 11, the
microcomputer 11 can determine whether a current exceeding the
current value x amperes has been detected by the current detection
circuit 14 for a period exceeding the prescribed time T based on
the number of times a current value exceeding the current value x
amperes was sampled and the sampling interval. Alternatively, the
microcomputer 11 may make the determination in S6 simply based on
the number of times a current value exceeding the current value x
amperes was continuously sampled.
[0047] When the microcomputer 11 determines that a large current
exceeding the prescribed value has been continuously applied to the
control FET 5 for the prescribed time T or greater (S6: YES),
indicating a continuous high load driving state, in S7 the
microcomputer 11 continues to perform chopping control by setting
the voltage applied to the control FET 5 to a voltage value c
volts, where the voltage value c volts is a lower effective voltage
than the voltage value b volts. Since the microcomputer 11 has
determined in S6 that the motor 3 was continuously driven, the
microcomputer 11 reads the third allowable power dissipation W3
from the storage device 12 for a continuous high load driving state
and calculates the voltage value c volts to be applied to the motor
3 based on the third allowable power dissipation W3 and the value
of current detected by the current detection circuit 14. The
voltage value c volts can be found from the following equation.
<Effective voltage c volts>=<third allowable power
dissipation W3>/<current value x amperes>
[0048] By setting the voltage value c volts to be applied to the
control FET 5 in this way, the microcomputer 11 can prevent power
dissipation from exceeding the preset third allowable power
dissipation W3 for a continuous high load driving state. In S7 the
microcomputer 11 next determines whether the current flowing to the
motor 3 exceeds the current value x amperes. The microcomputer 11
performs this determination by comparing the detected current value
outputted from the current detection circuit 14 with the current
value x amperes stored in internal RAM. If the microcomputer 11
determines that the current applied to the motor 3 exceeds the
current value x amperes (S8: YES), the microcomputer 11 returns to
S7 and continues to perform chopping control by setting the
effective voltage to be applied to the control FET 5 to a voltage
value d volts lower than the voltage value c volts. In order to
drop the effective voltage to the voltage value d volts, the OFF
duration of the control FET 5 must be shorter than that for the
voltage value c volts.
[0049] While the invention has been described in detail with
reference to specific embodiments thereof, it would be apparent to
specific embodiments thereof, it would be apparent to those skilled
in the art that many modifications and variations may be made
therein without departing from the spirit of the invention, the
scope of which is defined by the attached claims. For example, in
the preferred embodiment the battery voltage detection circuit 13
shown in FIG. 1 detects the battery voltage detection circuit 13
shown in FIG. 1 detects the battery voltage of the lithium-ion cell
set 6 through an accrual measurement. However, if the battery pack
2 has a built-in identification resistor representing ID data for
its internal cell set, the microcomputer 11 can determine the
battery voltage by reading this ID data. With this configuration,
the battery voltage detection circuit 13 provided in the power tool
1 of the embodiment shown in FIG. 1 may be omitted, thereby
eliminating power dissipation of the lithium-ion battery 6 caused
by the battery voltage detection circuit 13. The identification
data described above includes the type of battery (e.g.,
lithium-ion battery cells or the like), and the number of
cells.
[0050] The above embodiment describes the case of using a battery
pack that outputs a higher voltage than that of a battery pack
specifically designed for use with the power tool 1. However, it is
also possible to use a battery pack that outputs a lower voltage
than that of the specialized battery pack. In this case, a
switching integrated circuit may be provided in the power tool 1
for boosting the battery voltage supplied from the battery pack
mounted on the power tool 1 through DC-DC conversion, and the
microcomputer 11 may control this boosted DC voltage.
[0051] Further, the present invention may be applied to a drive
unit employing an FET for driving a motor, such as brushless DC
motor, and the same effects of the invention within the scope of
power dissipation in the motor and FET can be obtained by
performing the same control described in the preferred
embodiment.
REFERENCE SIGNS LIST
[0052] 1 power tool [0053] 2 battery pack [0054] 3 motor [0055] 4
trigger switch [0056] 5 control FET [0057] 6 lithium-ion battery
[0058] 7 battery protection circuit [0059] 8 thermistor [0060] 9
battery shutdown terminal [0061] 10 three-terminal regulator [0062]
11 microcomputer [0063] 12 storage device [0064] 13 battery voltage
detection circuit [0065] 14 current detection circuit [0066] 15
shutdown circuit [0067] 16 chopper circuit [0068] R1-R10 resistors
[0069] C1, C2 capacitors for preventing circuit oscillation
* * * * *